The invention relates to a luminescent Eu.sup.3+ -activated alkaline earth metal-rare earth metal silicate. The invention also relates to a luminescent screen comprising such a luminescent silicate and to a low-pressure mercury vapour discharge lamp provided with such a luminescent screen.
Luminescent silicates of an alkaline earth metal and of a rare earth metal, which silicates also comprise a halogen, are known from U.S. Pat. No. 3,650,974. These halosilicates have the crystal structure of the mineral apatite. A calcium-yttrium fluorosilicate is shown as an example of an Eu.sup.3+ -activated material. This Eu.sup.3+ -activated silicate has the characteristic red Eu.sup.3+ emission upon excitation by ultraviolet radiation. The light output of this silicate is, however, considerably smaller than that of Eu.sup.3+ -activated Y.sub.2 O.sub.3. Eu.sup.3+ -activated Y.sub.2 O.sub.3 is a very efficient red luminescing material which is frequently used in practice but which has the drawback of being very expensive. Therefore constant research is being done for less expensive efficient red luminescing materials.
It is to be noted that a rare earth metal silicate defined by the formula Ba.sub.2 Nd.sub.2 Si.sub.4 O.sub.13 is known from Dokl. Akad. Nauk SSSR, 285(1), pages 124-128, 1985. This silicate has a triclinic crystal structure. Nothing is known about possibilities of activating this silicate for the purpose of obtaining a luminescent material.